(1) Field of the Invention
This invention relates to an improvement in or relating spin drier to remove water droplets stuck on surfaces of semiconductor materials (wafers) by centrifugal force and to dry the same.
(2) Description of the Prior Art
This type of spin drier comprises a casing and a rotor rotated in the casing, and the rotor has usually four air-guides formed of a curved thin plate, said air-guides being mounted on the circular base plate of the rotor. Wafers to be dried are housed in a carrier, the carrier being also housed in a cradle to be set in position between air-guides in the rotor. There is provided a lid on the casing, which lid is positioned above the rotor and has a suction port formed therein, and the peripheral wall of the casing has an exhaust port formed at a suitable position thereof. Centrifugal force due to an acceleration at the starting of rotation of the rotor will scatter away and remove most of water droplets stuck on wafer surfaces, and after the rotary speed of the rotor attains to the specified one, wafers are dried by air stream flowing from the suction port through the central zone in the rotor to along the peripheral wall of the casing and finally to the exhaust port.
Usually, in commonly accepted technique, the rotor which is made of steel rotates at about 1200-1400 rpm. Accordingly, in case of a rotor of 600 mm diameter, the rotor has its weight of about 22 Kg and due to the weight, the acceleration time, namely the time to attain to the specified speed at the starting, requires about 20 seconds. In case that there is extremely small quantity of water droplets stuck on wafer surfaces, surface tension and adhesion of water droplets are larger than centrifugal force of the same. Thus, such acceleration time (rising time to the specified speed) of 20 seconds cannot remove the described small quantity of water droplets, which results in occurrence of water marks. Occurrence of water marks will cause natural oxidization which leads to deterioration in acid-proof characteristics. And, this lowers the rate of yield and thus raises production costs.